The actor, 47, seemed to have trouble walking while leaving his hotel for the UNICEF Masquerade ball in West Hollywood on Saturday, according to video obtained by TMZ.
Photos from the night show the star wearing a dark suit and a Halloween skull mask over his face.
“Sobriety is difficult and elusive for everyone struggling with addiction,” a friend of Affleck tells PEOPLE. “Ben has acknowledged he’s going to slip up from time to time. It was never as if this was simply behind him.”
Hours earlier, the actor addressed having been in recovery for over a year on social media after cracking a joke about his personal life.
“But let’s be serious for a moment and talk about something that is actually important,” he wrote. “I have been in recovery for over a year and part of that is helping out others. @themidnightmission is an incredible organization that helps those in need with housing, training, development and recovery.”
“I’m making a donation today because there are people battling addiction every day that don’t have the resources and need help,” he continued.
Affleck, who’s been in rehab multiple times, entered rehab again last August.
“This week I completed a forty-day stay at a treatment center for alcohol addiction and remain in outpatient care,” Affleck revealed in the Oct. 4 post. “Battling any addiction is a lifelong and difficult struggle.”
“Because of that, one is never really in or out of treatment. It is full-time commitment,” he continued “I am fighting for myself and my family.”
RELATED VIDEO: Ben Affleck Reveals He Completed 40-Day Rehab Stay: ‘I Am Fighting for Myself and My Family’
“Being an alcoholic, it’s part of my life, it’s something that I deal with,” said the actor. “It doesn’t have to subsume my whole identity and be everything but it is something that you have to work at.”
“I had a problem and I really want to address it and I take some pride in that,” he added. “It’s about yourself, your life, your family… we encounter these kinds of hurdles and we have to deal with them.”
“He is very open about how challenging it is for him and works on his sobriety daily,” a source previously told PEOPLE “Ben thinks of alcoholism as a disease—you shouldn’t be ashamed of a disease.”
Affleck is “very serious about his sobriety. It’s definitely not easy for Ben,” the insider added.